The present invention relates to a gray scale reducing method, an image displaying method, and an image displaying apparatus. More specifically, it relates to a gray scale reducing method for favorably reducing the number of gray levels with no use of a gray scale conversion table and to a method and an apparatus for speciously reproducing a fine image of which the number of gray levels is greater than that of a display panel.
FIG. 9 is a block diagram of a conventional liquid crystal display.
The liquid crystal display 500 comprises a 1024-to-1021 gray scale conversion table 51 for converting an image data A(1024) of 1024 gray levels (=10 bits) received from, e.g., a computer (not shown) into an image data Y(1021) of 1021 gray levels, a 256 gray scale image data sample storage 2 where image data samples of 256 gray levels are stored as sets of four different 256 gray scale images (=8 bits) for speciously reproducing an image of 1021 gray levels, a random number generator circuit 3 for generating a random number N to select and release a set of image data D1 to D4 of 256 gray levels at each four frames in response to the image data Y(1021) of 1021 gray levels received from the 256 gray scale image data sample storage 2, and a 256 gray scale monochrome liquid crystal display panel 4 for displaying images of 256 gray levels determined by the 256 gray scale image data D1 to D4. The image to be reproduced shall be a monochrome image.
FIG. 10 is a diagram showing an arrangement of the 1024-to-1021 gray scale conversion table 51.
The image data A(1024) of 1024 gray levels stored at the (left) entry column consists of a binary 10-bit value ranging from xe2x80x9c0000000000xe2x80x9d and xe2x80x9c0000000001xe2x80x9d, to xe2x80x9c1111111111xe2x80x9d equivalent from 0 to 1023 (of the decimal notation).
The image data Y(1021) of 1021 gray levels at the (right) entry column consists of a binary 10-bit value which corresponds to a 1024 gray scale image ranging from 0 to 1020 (of the decimal notation). As the gray levels are reduced by three, three entries of the 10-bit value are overlapped in the storage.
FIG. 11 is an explanatory view showing the principle of speciously reproducing an image of 1021 gray levels with the use of four frames of 256 gray levels.
It is assumed that the frames F1, F2, F3, and F4 are displayed at short intervals of {fraction (1/60)} the second and their gray level of 256 gray scale at a given pixel is varied from L1, L2, and L3 to L4. The gray level at the pixel is apparently equal to a level (L1+L2+L3+L4) of the 1021 (=2255xc3x974+1) gray scale. When L1=63, L2=63, L3=63, and L4=64, the level 253 of the 1021 gray scale can speciously be reproduced. This technique for reproducing an intermediate tone with the use of a series of frames is known as interframe error diffusion or Frame Rate Control.
Accordingly, as each set of the four different image data samples of 256 gray levels stored in the 256 gray scale image data sample storage 2 (FIG. 4) are equal to an image data Y(1021) of 1021 gray levels, they can speciously be reproduced as a 1021 gray scale mono chrome image.
The random number N provided for selection of a set of the 256 gray scale image data samples is intended to minimize the generation of xe2x80x9cflickerxe2x80x9d and stripe noises which are derived from the regularity of a change in the grays level at each frame.
The conventional liquid crystal display 500 requires a memory for storage of 10 kilobits (=1024xc3x9710 bits) as the 1024-to-1021 gray scale conversion table 51 and will thus be increased in the overall cost.
Also, if the random number N is low in the accuracy (i.e. a particular pattern appears at very short intervals and its value is repeated at high frequency), the effect of xe2x80x9cflickerxe2x80x9d and stripe noises may increase thus allowing an observer to have an abnormal impression.
It is hence a first object of the present invention to provide a gray scale reducing method for favorably reducing the number of gray levels with no use of a gray scale conversion table.
It is a second object of the present invention to provide a method and an apparatus for speciously reproducing a fine image of which the number of gray levels is greater than that of a display panel.
As a first aspect of the present invention, a gray scale reducing method is provided comprising a step of converting a bit stream input (axcex1xe2x88x921, axcex1xe2x88x922, axcex1xe2x88x923, . . . , a0) of 2xcex1 gray levels expressed by A=xcex1xe2x88x921xc3x972xcex1xe2x88x921+axcex1xe2x88x9222xc3x972xcex1xe2x88x922+axcex1xe2x88x923xc3x972xcex1xe2x88x923+ . . . +a0xc3x9720[xcex1xe2x89xa72] into a bit stream output of (2xcex1xe2x88x922xcex1xe2x88x92xcex2+1) gray levels expressed by Y=axcex1xe2x88x921xc3x972xcex1xe2x88x921+axcex1xe2x88x922xc3x972xcex1xe2x88x922+axcex1xe2x88x923xc3x972xcex1xe2x88x923+ . . . +a0xc3x9720xe2x88x92(axcex1xe2x88x921xc3x972xcex1xe2x88x92xcex2xe2x88x921+axcex1xe2x88x922xc3x972xcex1xe2x88x92xcex2xe2x88x922+ . . . +axcex2xc3x9720)[xcex1 greater than xcex2xe2x89xa71].
The gray scale reducing method of the first aspect enables a simple arithmetic operation of subtracting the upper (xcex1xe2x88x92xcex2) bits from the input A of 2xcex1 gray levels as a bit stream (axcex1xe2x88x921, axcex1xe2x88x922, axcex1xe2x88x923, . . . , a0) to determine a bit stream output Y of (2xcex1xe2x88x922xcex1xe2x88x92xcex2+1) gray levels with no use of any gray scale conversion table. Since the inputs A to be converted into the identical output Y are equally dispersed at 2xcex1xe2x88x92xcex2xe2x88x921 different locations, its conversion can be prevented from biasing and thus improved in the accuracy. More particularly, when xcex1=10 and xcex2=8, a bit stream of 1024 (=210) gray levels can accurately be converted into a bit stream of 1021 (=210xe2x88x92210xe2x88x928+1) gray levels.
As a second aspect of the present invention, an image displaying method is provided comprising steps of converting an image data by the gray scale reducing method of the first aspect from a bit stream of 2xcex1 gray levels to a bit stream of (2xcex1xe2x88x922xcex1xe2x88x92xcex2+1) gray levels, and speciously reproducing by interframe error diffusion a fine image of which the gray level is equal to that of the converted bit stream.
The image displaying method of the second aspect enables to speciously reproduce by interframe error diffusion an image of which the number of gray levels is favorably reduced by the gray scale reducing method of the first aspect.
As a third aspect of the present invention, an image displaying method is provided comprising a step of controlling the gray level in each of m segments (mxe2x89xa72) of one pixel on the frame-by-frame basis to speciously reproduce with the use of plural frames an image of which the number of gray levels is greater than that of a display panel which is capable of varying the gray level in each segment.
The image displaying method of the third aspect enables to change the gray level at each segment of a single pixel for spatially modulating the luminance of the pixel in one frame, thus displaying the number of gray levels of the pixel which is greater than that of the segments. More particularly, when one pixel consists of three segments, the gray level in one or two of the three segments can be declined to be lower by one step than that of the other segments or segment to reproduce an intermediate tone at steps of ⅓ the gray level. Accordingly, during the process of interframe error diffusion to speciously reproduce an image of which the number of gray levels is increased, the gray level of each pixel can be minimized in the difference between frames hence significantly attenuating the generation of xe2x80x9cflickerxe2x80x9d and stripe noises in the image.
As a fourth aspect of the present invention, an image displaying apparatus is provided comprising a gray scale reducing means for converting a bit stream (axcex1xe2x88x921, axcex1xe2x88x922, axcex1xe2x88x923, . . . , a0) of 2xcex1 gray levels of image data expressed by A=xcex1xe2x88x921xc3x972xcex1xe2x88x921+axcex1xe2x88x922xc3x972xcex1xe2x88x922+axcex1xe2x88x923xc3x972xcex1xe2x88x923+ . . . +a0xc3x9720[axe2x89xa72] into a bit stream of (2xcex1xe2x88x922xcex1xe2x88x92xcex2+1) gray levels of image data expressed by Y=axcex1xe2x88x921xc3x972xcex1xe2x88x921+axcex1xe2x88x922xc3x972xcex1xe2x88x922+axcex1xe2x88x923xc3x972xcex1xe2x88x923+ . . . +a0xc3x9720xe2x88x92(axcex1xe2x88x921xc3x972xcex1xe2x88x92xcex2xe2x88x921+axcex1xe2x88x922xc3x972xcex1xe2x88x92xcex2xe2x88x922+ . . . +axcex2xc3x9720)[xcex1 greater than xcex2xe2x89xa71], a displaying panel of which the number of gray levels for reproducing each frame of the image on a pixel-by-pixel basis is smaller than 2xcex1, and an image display controlling means for displaying on the display panel a series of images which have speciously been reproduced by interframe error diffusion and have corresponding gray levels determined by the converted bit streams.
The image displaying apparatus of the fourth aspect allows the image displaying method of the second aspect to be satisfactorily performed.
As a fifth aspect of the present invention, an image displaying apparatus is provided comprising a displaying panel which is capable of varying the gray level in each of m segments (mxe2x89xa72) of one pixel, and an image display controlling means for controlling the gray level of each segment on the frame-by-frame basis to speciously reproduce an image of which the number of gray levels is greater than that applicable in the segment at each frame.
The image displaying apparatus of the fifth aspect allows the image displaying method of the third aspect to be satisfactorily performed.
As a sixth aspect of the present invention, an image displaying apparatus is provided comprising a gray scale reducing means for converting a bit stream (axcex1xe2x88x921, axcex1xe2x88x922, axcex1xe2x88x923, . . . , a0) of 2xcex1 gray levels of image data expressed by A=axcex1xe2x88x921xc3x972xcex1xe2x88x921+axcex1xe2x88x922xc3x972xcex1xe2x88x922+axcex1xe2x88x923xc3x972xcex1xe2x88x923+ . . . +a0xc3x9720[xcex1xe2x89xa72] into a bit stream of (2xcex1xe2x88x922xcex1xe2x88x92xcex2+1) gray levels of image data expressed by Y=axcex1xe2x88x921xc3x972xcex1xe2x88x921+axcex1xe2x88x922xc3x972xcex1xe2x88x922+axcex1xe2x88x923xc3x972xcex1xe2x88x923+ . . . +a0xc3x9720xe2x88x92(axcex1xe2x88x921xc3x972xcex1xe2x88x92xcex2xe2x88x921+axcex1xe2x88x922xc3x972xcex1xe2x88x92xcex2xe2x88x922+ . . . +axcex2xc3x9720)[xcex1 greater than xcex2xe2x89xa71], a displaying panel for reproducing 2xcex2 gray levels in each of m segments (mxe2x89xa72) of one pixel, a gray level calculating means for sunning the upper xcex2 bits of the converted bit stream and an adjusting gray level predetermined for each segment by the lower (xcex1xe2x88x92xcex2) bits of the bit stream to determine the gray level in the segment at each of frames to be subjected to interframe error diffusion, and an image display controlling means for controlling the gray level in the segment of the display panel on the frame-by-frame basis.
The image displaying apparatus of the sixth aspect enables to change the gray level at each segment of a single pixel, hence minimizing the different in the gray level of the pixel between frames and significantly attenuating the generation of xe2x80x9cflickerxe2x80x9d and stripe noises in the image.
Also, the gray level of each segment of the frames can favorably be calculated by assigning a gray level of the upper xcex2 bits of the bit stream of (2xcex1xe2x88x922xcex1xe2x88x92xcex21) gray levels determined by the gray scale reducing method of the first aspect to the segment as the basic gray level in common with the segment of each of pixels at the frames to be subjected to interframe error diffusion and adding with an adjusting gray level predetermined and assigned to the lower (xcex1xe2x88x92xcex2) bits. This can requires a less storage capacity of the gray level data as compared with preliminary storage of all the (2xcex1xe2x88x922xcex1xe2x88x92xcex2+1) gray levels of each segment, thus contributing the reduction of the overall cost.
As a seventh aspect of the present invention, the image displaying apparatus of the sixth aspect is modified further comprising an adjusting gray level sample storing means for saving samples of the adjusting gray level predetermined and assigned to each segment so that the average of gray levels of each pixel at the 2xcex1xe2x88x92xcex2 frames for the error diffusion calculated by the gray scale reducing means is the upper xcex2 bits plus the least (xcex1xe2x88x92xcex2) bits/2xcex1xe2x88x92xcex2, and a randomly selecting and releasing means for randomly selecting and releasing any of the samples of the adjusting gray level on the frame-by-frame basis.
The image displaying apparatus of the seventh aspect enables to randomly select and release on the frame-by-frame basis a sample of the adjusting gray levels assigned to the segments so that the average gray level of each pixel in 2xcex1xe2x88x92xcex2 error diffusion frames is equal to the upper xcex2 bits plus {the lower (xcex1xe2x88x92xcex2) bits/2xcex1xe2x88x92xcex2}, thus allowing each frame of the image of which the gray level is very close to the average gray level to be reproduced in case that a number of the error diffusion frames are aligned along the time base. The random selection is intended for minimizing the generation of xe2x80x9cflickerxe2x80x9d and stripe noises derived from the regularity of changes in the gray level of each segment.
More particularly, when xcex1=10 and xcex2=8, the average gray level of each pixel in four of the error diffusion frames is equal to a sum of 256 gray levels expressed by the upper eight bits of a bit stream of 1021 gray levels and four gray levels expressed by the lower two bits/4 and can hence be favored to speciously reproduce an image of 1021 gray levels.
As an eighth aspect of the present invention, the image displaying apparatus of any of the fourth to seventh aspects is modified wherein the display panel is a monochrome liquid crystal display panel.
The image displaying apparatus of the eighth aspect enables to display an image on the monochrome liquid crystal display and can thus be favored to speciously reproduce a monochrome image of multiple gray levels through interframe error diffusion.